Vineyard Deficit Irrigation Strategies
Regulated Deficit Irrigation (RDI)
Regulated deficit irrigation (RDI) is a term used for the practice of regulating or restricting the application of irrigation water until a level of vine water stress is attained in order to improve fruit quality, reduce water consumption, and control canopy growth. RDI can be a consistent reduction (i.e., consistent reduction of planned irrigation volumes over the entire season) or at specific periods (e.g., fruit set to véraison) of the growing season to induce the desired vine response.
Timing of Water Deficits
RDI can be applied to wine grapes either from fruit set to véraison, from véraison to harvest or over both periods. The effects of a moderate level of stress from RDI during each of these periods compared with full irrigation throughout the growing season can be summarized as follows.
Fruit Set to Véraison
The RDI period starts when there are no remaining signs of flowering and during the early stages of fruit set.
Véraison to Harvest
Water deficits during this period have been shown to increase wine color, aroma, and flavor. Depending on the water deficit selected, berry size can be significantly reduced and subsequently yield when compared to well irrigated vines.
Fruit Set to Harvest
From fruit set to harvest, water deficits will reduce vegetative growth and yield, maintain or increase the concentration of soluble solids, reduce titratable acidity, and marginally increase pH.
Postharvest Irrigation Deficits
A continued or increasing water deficit following harvest provides little or no benefit to the vines or next year’s crop.
Important Considerations Using RDI Strategy
In some situations, it may be necessary to grow cover crop to reduce the amount of available water to the vines.
Soil type (texture) has to be considered when implementing a deficit irrigation strategy as does other soil attributes such as root zone depth, rates of infiltration, and the total water available to vine roots. Lighter soil types (sands, sandy loams) are the easiest soils with which to implement deficit irrigation practices since the soil moisture levels are controlled quite easily (depending on weather of course).
The occurrence of a large amount of rain during the RDI period will limit the amount of water stress occurring to the vine and may render RDI ineffectual.
Rootstock varieties vary in their response to soil moisture levels, so experience is needed in setting irrigation thresholds.
Red grape varieties often experience improved color as a result of deficit irrigation practices, in part, simply due to smaller berry size, which increases the skin:pulp ratio.
Low Vigor Vineyards
Extremely low vigor vineyards with barely sufficient leaf area to sustain vine growth and mature fruit are not appropriate candidates for the regulated deficit irrigation.
Mid-day Leaf Water Potential (MDLWP)
Mid-day leaf water potential (MDLWP) of -10 to -13 bars is a typical irrigation start point for white wine grapes when implementing a RDI program.
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